Differential Gene Expression
If the genome is the same in all somatic cells within an
organism (with the exception of the above-mentioned
lymphocytes), how do the cells become different from one
another? If every cell in the body contains the genes for
hemoglobin and insulin proteins, how are the hemoglobin
proteins made only in the red blood cells, the insulin
proteins made only in certain pancreas cells, and neither
made in the kidney or nervous system? Based on the
embryological evidence for genomic equivalence (and on
bacterial models of gene regulation), a consensus emerged in
the 1960s that cells differentiate through differential gene
expression. The three postulates of differential gene
expression are as follows:
the hundreds of different cell types in the body. The question
then became, How does this differential gene expression
occur? The answers to that question will be the topic of the
next chapter. To understand the results that will be
presented there, however, one must become familiar with
some of the techniques of molecular biology that are being
applied to the study of development. These include
techniques to determine the spatial and temporal location of
specific mRNAs, as well as techniques to determine the
functions of these messages.
1. Every cell nucleus contains the complete genome
established in the fertilized egg. In molecular terms, the
DNAs of all differentiated cells are identical.
2. The unused genes in differentiated cells are not destroyed
or mutated, and they retain the potential for being expressed.
3. Only a small percentage of the genome is expressed in
each cell, and a portion of the RNA synthesized in the cell is
specific for that cell type.
The first two postulates have already been discussed. The
third postulate—that only a small portion of the genome is
active in making tissue-specific products—was first tested in
insect larvae. Fruit fly larvae have certain cells whose
chromosomes become polytene. These chromosomes,
beloved by Drosophila geneticists, undergo DNA replication
in the absence of mitosis and therefore contain 512 (29), 1024
(210), or even more parallel DNA double helices instead of
just one (Figure 4.13A,Figure 4.13B). These cells do not
undergo mitosis, and they grow by expanding to about 150
times their original volume. Beermann (1952) showed that
the banding patterns of polytene chromosomes were
identical throughout the larva, and that no loss or addition of
any chromosomal region was seen when different cell types
were compared. However, he and others showed that in
different tissues, different regions of these chromosomes
were making organ-specific RNA. In certain cell types,
particular regions of the chromosomes would loosen up,
“puff” out, and transcribe mRNA. In other cell types, these
regions would be “silent,” but other regions would puff out
and synthesize mRNA.
The idea that the genes of chromosomes were differentially
expressed in different cell types was confirmed using DNARNA hybridization (Figure 4.13C). This technique involves
annealing single-stranded pieces of RNA and DNA to allow
complementary strands to form double-stranded hybrids.
While some mRNAs from one cell type were also found in
other cell types (as expected for mRNAs encoding enzymes
concerned with cell metabolism), many mRNAs were found
to be specific for a particular type of cell and were not
expressed in other cell types, even though the genes
encoding them were present (Wetmur and Davidson 1968).
Thus, differential gene expression was shown to be the way a
single genome derived from the fertilized egg could generate
Figure 4.13. Polytene chromosomes. (A) Polytene chromosomes
from the salivary gland cells of Drosophila melanogaster. The four
chromosomes are connected at their centromere regions, forming a
dense chromocenter (arrowhead). The DNA has been stained red
with propidium iodide stain. The yellow stain represents the
binding of a particular protein to the DNA. This protein is
involved in compartmentalizing the regions of the chromosome so
that the activation of one gene does not cause the activation of its
neighbors. (B) Electron micrograph of a small region of a
Drosophila polytene chromosome. The bands (dark) are highly
condensed compared with the interband (lighter) regions. (C)
Hybridization of a yolk protein mRNA with the polytene
chromosome of a larval Drosophila salivary gland. The dark grains
(arrow) show where the radioactive yolk protein message has bound
to the chromosomes. Note that the gene for the yolk protein is
present in the salivary gland chromosomes, even though yolk
protein is not synthesized there. (A courtesy of U. K. Laemmli; B
from Burkholder 1976, courtesy of G. D. Burkholder; C from
Barnett et al. 1980; photograph courtesy of P. C. Wensink.)